This invention relates to a method and apparatus for the measurement of the velocity of fluids, and particularly to such method and apparatus which utilize the transmission of acoustic waves through the moving fluid.
It is known in the art to determine the velocity of a fluid by transmitting acoustic waves through the moving fluid from a transmitter of acoustic waves to a receiver of acoustic waves and deriving from the received acoustic waves indications of the velocity of the fluid; the acoustic waves are transmitted in a direction which has at least a substantial component parallel to the direction of fluid flow, and preferably such transmissions are effected both in the upstream and downstream directions and the information contained in the received waves combined to obtain the desired fluid velocity indications. Thus while transmission in one direction alone will provide information useful for some purposes, the use of upstream and downstream transmissions increases the accuracy greatly since it eliminates the factor of the velocity v.sub.0 of the acoustic waves in still fluid and is not dependent thereon, which velocity v.sub.0 typically may vary substantially with such factors as temperature and composition of the fluid, including for example its content of gaseous, liquid or solid materials.
In order to accomplish the upstream and downstream type of fluid velocity measurement with the best accuracy, it is known to utilize two acoustic transducers between which the fluid flows, each such transducer serving alternately as a transmitter and a receiver of acoustic waves. More particularly, first one transducer acts as a transmitter to transmit acoustic waves to the other transducer acting as a receiver, and then the other transducer is caused to transmit acoustic waves to the first transducer which then acts as a receiver. In this way, undesired differences in upstream and downstream acoustic wave delays due to use of different transducers for the two directions of transmission are greatly mitigated.
Known types of acoustic fluid velocity measuring systems include the Doppler frequency system which detects apparent changes in acoustic wave frequency due to fluid motion, but which has the drawback that particles must normally be present in the fluid in order to develop a suitable signal, and that accuracy is usually limited to about 5% at best.
It is also known to measure fluid velocity by transmitting a short acoustic-wave pulse through the fluid to the receiver, and measuring directly the propagation delay of the impulse in travelling from transmitter to receiver; this again may be done in both the upstream and downstream directions. Typically the pulse delay is converted to a frequency equal to 1/delay by means of a feedback circuit, and the difference between the resultant "upstream" and "downstream" frequencies is used as a measurement of flow velocity which is substantially independent of sound velocity in the still fluid. A limitation of this method is that in order to get the necessary resolution of the pulses, the transducer must operate in the megahertz region where propagation losses are undesirably high in liquids, and so high in gases as to make the system virtually unuseable. Also, the necessity for transmitting and receiving a narrow pulse limits the energy available for detection, and for best results requires use of transducers with very wide bandwidths.
Phase comparison methods are also known to measure the fluid velocity. In such systems the phase of the transmitted signal is compared to that of the received signal; typically the phase delay for downstream propagation is compared with the phase delay for upstream propagation to give a sensitive measurement of fluid flow rate. This technique has the advantage that greater signal power can be transmitted then when only a narrow transmitted pulse is used, providing a better signal to noise ratio. Also, the frequencies employed are lower than in the narrow-pulse system, making operation in gas practical, with generally less attenuation in any medium.
However, known phase comparison methods depend on knowledge of ambient sound velocity v.sub.0, which varies widely with temperature and type of fluid. Also, accuracy will generally suffer if there are any significant reflections of the acoustic waves from the transducers or from the surrounding walls of the fluid chamber. Particularly troublesome are triple reflections directly off the transducer faces themselves. Standing waves caused by such reflections can often affect the accuracy by 50% or more; also, linearity as the function of fluid velocity is affected by such standing waves.
It is therefore an object of the present invention to provide a new and useful method and apparatus for measuring the velocity of a fluid.
Another object is to provide such method and apparatus which retain the principal advantages of the phase comparison methods of measurement previously known, but avoid or greatly reduce the drawbacks associated with previously known phase comparison methods.
Still another object is to provide such method and apparatus which is accurate over a wide range of temperatures and fluid types and compositions, and at the same time provides accurate measurement.